What breed are they? Cliping one or both wings? The heavier, the lower the roost needs to be. Will there be a ladder up or just the lone bar? I'll say 4ft for the fattest birds, Most 5ft, but if you have 'flighty' banties or Dutch, 8ft would still work.
it depends on if there is enough room for them to fly back down. You don't want them smashing into walls when they make their decent.
Mine will roost at 7 feet if given the chance, but they also have about 8 feet of space between roost and wall for a safe landing. My cockerel once roosted outside in a tree which was about 10 feet high. He's about 4 lbs.
as posted above some chickens prefer not to roost too high, while others will roost as high as you let them. A ladder or ramp would be recommended for really high roosts.